a) Field of the Invention
This invention relates to a method of brazing or welding a ceramic body, such as a saw tip or other object, to a substrate such as a saw blade tip. The present invention also relates to the ceramic body itself which is processed in a particular manner so that it can be brazed or welded to a substrate, and also to an article made up of such a ceramic body joined to a substrate.
b) Background Art
Ceramics have attributes such as high wear and corrosion resistance as well as stability at high temperature that make their use desirable in applications such as saws, tools and jet engine exhausts, and in other applications where similar conditions exist.
The very qualities that make ceramics desirable for use in these applications also make them very hard to use. Resistance to wear, heat and corrosion make them very difficult to braze or weld successfully. Currently it is possible to braze ceramics using special alloys in special atmospheres and with elaborate preparation. Preparing ceramics for plating and brazing is well known in industries such as electronics and aerospace. The drawbacks have been that the special atmospheres and special alloys also made the joining very expensive in terms of materials, quipment and overall processing costs. In addition the epoxies and alloys suitable for brazing ceramics are typically too weak and break down at temperatures too low to make their use, practical in many applications. Active metal alloys, catalytic surface treatment, special atmospheres and multi-step processing all add to the cost of the processing in an extremely price sensitive industry.
Brazing is commonly defined as a process for joining two dissimilar materials at a temperature in the range of, or above, 800.degree. F. The brazing alloys which are commonly used are silver based because of their strength, impact resistance and corrosion resistance, and quite commonly in the temperatures in the range 1150.degree. F. to 1350.degree. F. are employed in the brazing process. The brazing can be accomplished by various heating means, such as utilizing oxyacetylene, induction brazing or resistance brazing. It is also possible that could be accomplished in an oven, or with plasma. The more common methods are simply to use an oxyacetylene torch or induction brazing.
Another process of joining two metals together is called welding where the two metal objects are placed in contact with one another without the use of a separate brazing alloy, and heat is applied so that the two metals flow together and upon cooling become joined to one another.
One application of brazing in connection with a ceramic like material is brazing tungsten carbide saw tips to a circular saw blade. The saw blade is made of a high quality steel, and plurality of notches are formed in the perimeter of the saw blade. The tungsten carbide tips are formed in the desired configuration, with these tips have a bonding surface matching the configuration of the notches in the saw blade.
The bonding surfaces of the notches in the saw blade and of the tips are properly prepared to remove contaminants, and a brazing compound is applied to the bonding surface of the tips.
The tips are placed in the notches of the saw blade, and heat is applied. The assembly of the saw blade with the tips is permitted to cool, to form a completed circular saw with the tips securely joined to the saw blade.
When the welding process is employed, the brazing compound is not used. In other respects, the welding process is similar to the brazing process.
The tungsten carbide tips are quite durable and are able to withstand relatively high temperatures. Thus, for high quality saws that need to operate in more severe environments (higher temperature, vibration, impacts, etc.), tungsten carbide tips have commonly been used.
It has long been recognized that saw tips made of cermet have certain qualities which surpass those of tungsten carbide. First, cermets are highly resistant to various corrosive materials. This makes use of cermets quite desirable when the saw blades are commonly used to cut cedar, spruce, hemlock and other green lumber, since these types of wood have acids that can corrode the saw blade, the saw tips and the bonding material. Further, a cermet saw tip, made (for example) from titanium carbonitride is able to maintain a sharper edge for as much as three times as long or longer than a comparable tungsten carbide saw tip. Also, a tip made of titanium carbonitride tip is able to withstand higher temperatures, and maintain a sharp edge.
However, in general it has not been possible to achieve the benefits which could be achieved by using saw tips made from a cermet material, mainly because of the difficulty of reliably bonding the cermet to the saw blade. In general the epoxies and alloys which are more suitable for brazing ceramics or cermets are too weak and also break down at temperatures too low to make them commercially practical for many applications.
Thus, when ceramics (including cermets) have been used, it is commonly in indexable or mechanically held applications in tools. For example, they can be used in a rotary cutting tool where the cutter inserts are placed in a pocket at the perimeter of the tool and mechanically held in place. However, to the best knowledge of the applicants, it has not been found to be commercially practicable to employ cermets where they must be brased or otherwise bonded to the saw blade or other substrate.